Why is "too many" superhero flicks in one year considered any more "oversaturation burnout" than "too many" horror films? Or "too many" bromances or rom-coms? Or "too many" cop dramas?

Because horror/romance/drama films don't have to make 300M to break even and be considered a success.

Also, while superhero films vary in tone and setting, horror films vary widely in terms of character types, story arcs and outcomes, as well as tone and setting. So, there are fewer rules, and even when those are broken, the film is lauded for being groundbreaking not sniped because "I didn't see enough horror tropes." So there's more variety in horror, same with romantic comedies, so much so that they have sub categories like bromances. This is in addition to if a film is "one film too many" the company loses something like 30M instead of 300M.

And there can be too many cop dramas, they too have limited character types and settings.